The invention relates to a piston stopper for a medicine injector and to a method for manufacturing a piston stopper.
Medicine injectors conventionally comprise a cylindrical receptacle for a liquid medicine which is injected out through a cannula by means of a piston. These pistons typically have a shaft and a piston stopper which seals the cylinder off against its inner wall. To this end, pistons are conventionally used which are provided with at least one sealing ring positioned in a circumferential groove. In known embodiments, the piston stopper consists of a relatively hard material and a sealing ring or sealing rings, which consist of a softer material for a better seal, are inserted into the circumferential grooves elastically like rubber. Thus, they are held in the grooves only by their own elasticity.
In the above, conventional embodiments, the sealing rings have to be installed in a separate step, which is therefore time-consuming. Also, the sealing rings' hold in the circumferential grooves is heavily dependent on the spring elasticity of the sealing rings, such that poorly produced sealing rings, or rings manufactured from a material which is already quite old or otherwise flawed, easily slip out of the grooves. Because the sealing rings are simply tensioned into the grooves, the danger always exists that they may leave the groove in which they are received or be badly twisted in their hold during use, giving rise to major leakage problems.
Attempt have been made to solve this problem. One attempt involves manufacturing piston stoppers from a soft solid-rubber material as complete components, wherein the sealing rings are merely formed as protruding circumferential attachments of the stopper material. Even this solution, however, involves problems, specifically when there is a high counter pressure from the liquid to be expelled. Such a high counter pressure can, for example, arise when a cannula is deformed or kinked. Because of the totally soft material of the stopper, the latter gives in such cases and the piston travels further forward in the outlet direction, wherein the liquid between the piston and the cylindrical inner wall is pressed backwards. In particular, when using injectors which are outwardly sealed, for example, so-called “pens,” there is the possibility that a patient would not even notice this process and believe that he or she has injected the medicine, when in fact it has flowed past the piston into the rear inner space of the cylinder.
Furthermore, piston stoppers in accordance with the prior art usually simply comprise threaded sections moulded into the stopper body, which do not establish an optimal connection with a plunger.